The present invention relates generally to packers utilized in subterranean wellbores and, in a preferred embodiment thereof, more particularly provides an improved backup shoe for utilization with a well packer.
A typical packer generally has one or more seal elements or "rubbers" which are utilized to provide a fluid-tight seal radially between a mandrel of the packer and casing into which the packer is disposed. Such a packer is commonly conveyed into the casing in a subterranean wellbore suspended from tubing extending to the earth's surface.
To prevent damage to the seal elements while the packer is being conveyed into the well, the seal elements are carried on the packer mandrel in a relaxed or uncompressed state in which they are radially inwardly spaced apart from the casing. When the packer is set, the seal elements are typically axially compressed between element retainers straddling the seal elements on the packer. This axial compression of the seal elements causes them to radially inwardly and outwardly extend, thereby sealing against the packer mandrel and against the casing.
The packer usually includes a number of slips which grip the casing and prevent movement of the packer axially within the casing after the packer has been set. Thus, if weight or fluid pressure is applied to the packer, the slips resist the axial forces on the packer produced thereby, and prevent axial displacement of the packer relative to the casing.
If, however, fluid pressure is applied to an annular space radially between the packer and the casing, and above or below the seal elements, which produces a differential pressure across the seal elements, the seal elements may be displaced axially into the annular space between the packer and the casing. Additionally, the seal elements may be displaced into voids, spaces, gaps, etc. on the packer, such as into a radial gap between the element retainer and the packer mandrel. Such displacements of the seal elements may be caused by fluid pressure acting on the seal elements, or may be caused by axial compression of the seal elements when the packer is set.
It is generally undesirable for the seal elements to displace into the abovedescribed gaps, voids, etc. for a number of reasons. For example, if the seal elements displace into the radial gap between the packer and the casing, and it is later desired to retrieve the packer from the well, the presence of the seal element material in the radial gap may make it difficult to axially displace the packer in the casing. More importantly, displacement of the seal element after the packer has been set usually compromises the ability of the seal elements to effectively seal between the casing and the packer mandrel.
Several attempts have been made to solve the problem of undesired seal element displacement or extrusion in a set packer. Typically, the solution involves placing one or more anti-extrusion rings, backup rings, or backup "shoes" axially straddling the seal elements, so that the seal elements must axially traverse the rings or shoes to extrude into the gaps or voids. However, most of these attempts have met with only limited success at best.
Several of the proposed solutions to the problem involve the use of multiple rings or shoes at each axial end of the seal elements. Unfortunately, this increases the cost of the packer, increases the number of parts to inventory, increases the risk that the packer will be incorrectly assembled, etc. Additionally, where it is desired to retrieve the packer after it has been set, multiple rings or shoes increase the risk that one or more of the rings will become jammed between the packer and casing, thereby making retrieval of the packer very difficult.
Some proposed solutions utilize rings or shoes which displace radially outward to contact the casing when the packer is set in an attempt to close off the radial gap between the packer and the casing. Most of these do not also displace radially inward to close off the gap between the element retainers and the packer mandrel, nor do they close off any other gaps or voids on the packer into which the seal elements may displace. This situation is particularly undesirable where seal element material is utilized, such as Aflas, that readily flows into gaps and voids under pressure. In addition, these solutions also suffer from the risk that they will not radially inwardly retract when it is desired to retrieve the packer from the casing.
Still other proposed solutions utilize rings or shoes which have an angled thick outer portion which must be forced outward to contact the casing. The thick outer portion is designed to withstand larger forces applied thereto than could be applied to thinner portions. Unfortunately, these solutions also require large setting forces to be applied to the packer in order to force the thick outer portions radially outward. In attempts to decrease these large setting forces, some have increased an axial distance between the thick outer portion and the seal element, and provided a ramp or extended lever arm therebetween. These attempts have indeed decreased the required setting force, but have consequently increased the axial travel required to compress the seal elements and set the packer.
From the foregoing, it can be seen that it would be quite desirable to provide a well packer with improved backup shoes which do not significantly increase the cost of the packer, increase the risk of improper assembly, require large setting forces or axial travel for setting of the packer, or hamper retrieval of the packer, but which prevent displacement of seal elements into a radial gap between the packer and casing in which it is set, prevent displacement of seal elements into a gap between element retainers and a mandrel of the packer, prevent displacement of seal elements into other voids and gaps on the packer, and which provide a relatively thick outer portion for resistance to relatively large forces applied thereto. It is accordingly an object of the present invention to provide such improved backup shoes and a well packer utilizing such backup shoes.